Phase 7c: string operators + and ==/!= (retire streq/sconcat)

Strings are values now: `a + b` concatenates and `a == b` / `a != b` compare by
content, replacing the 605 `sconcat(...)` / `streq(...)` calls that made the
compiler read like C.

  streq(name, "let")            -> name == "let"
  sconcat("load ", reg)         -> "load " + reg
  sconcat(a, sconcat(b, c))     -> a + b + c

Implementation: emit_bin gains a string path. Strings are pointer-typed, so any
`+` with a pointer operand concatenates and `==`/`!=` between pointers compares
content — except when one side is the `null` literal, which stays a pointer
identity test (the only two kinds of pointer `==` in the codebase). Both call a
small hand-written IR prelude, @fn_str_eq / @fn_str_concat, emitted once into any
program that uses string ops (so it works for tools, games and the compiler with
no runtime-splice dependency and no duplicate symbols).

Delivered as two reseeds: (A) add the operators + prelude with the full
pointer-aware dispatch, keeping streq/sconcat; (B) migrate every call site
(354 lines, via a string-literal-safe balanced-paren script that leaves the
function definitions alone) and delete streq/sconcat. examples/strings.ludic +
a test.sh smoke (prints 1 2 3 4 5) guard it.

Reseeded (21890 lines); C-free fixpoint holds; goldens byte-identical; 18/18;
vocab + doc-fences clean.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

selfhost/str.ludic

@@ -2,18 +2,6 @@
 # slicing out substrings, and integer<->text. All over raw NUL-terminated
 # byte buffers reached with peek8/poke8.
 
-fn streq(a: ptr, b: ptr) -> bool {
-  var i = 0
-  while true {
-    let ca = peek8(a, i)
-    let cb = peek8(b, i)
-    if ca != cb { return false }
-    if ca == 0 { return true }
-    i = i + 1
-  }
-  return false
-}
-
 fn slen(s: ptr) -> int {
   var n = 0
   while peek8(s, n) != 0 { n = n + 1 }